1. Field of the Invention
The present invention relates to a magnetic recording medium such as magnetic tape, more particularly to a magnetic recording medium comprising a magnetic layer formed by spreading a magnetic coating solution that mainly comprises a ferromagnetic powder and a binder, and a high density-recording magnetic recording medium having a hexagonal ferrite incorporated in a magnetic layer, particularly suitable for use in a system using an MR head utilizing a magnetoresistance effect for reproduction.
2. Background Art
A magnetic recording technique has found wide application to video, computer, etc. because it has excellent advantages which are not obtained in other recording methods, that is, it allows repeated use of medium, it provides easy electronization of signal to allow system construction by the combination with peripheries and it allows easy correction of signal.
In order to meet demands such as reduction of size of apparatus, enhancement of the quality of recorded/reproduced signal, prolongation of recording and rise of recording capacity, it has always been desired to provide recording medium with higher recording density, reliability and durability. For example, in order to cope with the practical use of digital recording system for the realization of enhancement of sound quality and image quality and the development of recording system corresponding to high definition TV, it has been more keenly desired to provide a magnetic recording medium which allows recording/reproduction of short wavelength signal and exhibits an excellent reliability and durability even when the speed thereof relative to head is raised. For computer use, too, it has been desired to develop a large capacity digital recording medium for storing an increasing amount of data. In the art of magnetic disc, too, it has been desired to provide a flexible disc with a larger capacity since the capacity of data to be processed is suddenly increasing today. A large capacity disc comprising a ferromagnetic metal powder excellent in high density recording properties has been put in practical use in the form of flexible disc having a density as high as 100 MB or more. However, a system having a larger capacity and a high transferring rate has been required.
In order to attain a high recording density in magnetic recording medium, there is a strong trend for more signals to be used in short wavelength range. When the length of the region in which signals are recorded stands comparison with the size of the magnetic material used, a definite magnetization transition state cannot be formed, substantially disabling recording. It is thus necessary to develop a magnetic material having a particle size which is sufficiently smaller than the shortest wavelength to be used. Therefore, size reduction of magnetic material has long been desired.
Referring to metal powders for magnetic recording, the particles are acicular and provided with anisotropy in profile to obtain the desired coercive force. It is well known to those skilled in the art that for high density recording, a ferromagnetic metal powder is needed to be finely divided so that the resulting magnetic recording medium has a smaller surface roughness. However, as more finely divided, the magnetic recording metal powder has a smaller acicularity ratio and thus can more difficultly provide the desired coercive force.
Magnetic tapes to be used in digital signal recording system are dedicated each for system. Magnetic tapes for so-called 3480 type, 3490 type, 3590 type, QIC type, D8 type and DDS type systems are known. Magnetic tapes for use in these systems each comprise a single magnetic layer having a thickness as relatively great as 2.0 to 3.0 μm comprising a ferromagnetic powder, a binder and an abrasive provided on one side of a non-magnetic support and a back coat layer provided on the other for preventing disturbance in winding or keeping a good running durability. However, the aforementioned relatively thick single magnetic layer is disadvantageous in that it undergoes self-demagnetization during recording and thickness loss causing output drop during reproduction.
It is known that the thickness of the magnetic layer is reduced to eliminate the drop of reproduced output due to the thickness loss of the magnetic layer. For example, JP-A-5-182178 discloses a magnetic recording medium comprising a lower non-magnetic layer having an inorganic powder dispersed in a binder and an upper magnetic layer having a thickness of 1.0 μm or less comprising a ferromagnetic powder dispersed in a binder provided on a non-magnetic support wherein the upper magnetic layer is formed while the non-magnetic layer is wet. For DLT type system, high capacity DDS type system and LTO system, coated magnetic recording medium comprising a magnetic layer having a thickness of from 0.2 to 0.4 μm provided on a lower non-magnetic layer has been put in practical use.
As magnetic recording medium such as video tape, computer tape and flexible disc there have heretofore been widely used those comprising a magnetic layer having a ferromagnetic iron oxide, Co-modified ferromagnetic ion oxide, CrO2, ferromagnetic metal powder, hexagonal ferrite or the like dispersed in a binder spread over a support. Among these magnetic materials, hexagonal ferrite is known to have excellent high density recording properties (see JP-A-60-157719, JP-A-62-109226 and JP-A-3-280215). JP-A-5-12650 discloses that when the thickness of a magnetic layer comprising the ferrite is predetermined to a range of from 0.1 μm to 0.6 μm and a non-magnetic layer having a thickness greater than the magnetic layer is provided interposed between the magnetic layer and the support, the resulting magnetic recording medium has improved surface properties, short wavelength output, erasing properties and durability. JP-A-5-225547 discloses a magnetic recording medium comprising a non-magnetic layer provided on a support and a magnetic layer having a magnetic powder having a size of 0.1 μm or less dispersed therein provided on the non-magnetic layer. It is thus disclosed that a magnetic recording medium having excellent high frequency range properties and good signal overwriting properties and durability is provided.
The recent trend is for more computer data recording systems to use a high sensitivity reproducing head (MR head) utilizing a magnetoresistance effect. Thus, systems providing a high S/N ratio have been developed. In these systems, the system noise is governed by noises derived from the magnetic recording medium used. In other words, it is essential that the magnetic recording medium for systems comprising MR head generate reduced level of noises. Further, it is necessary that both the desired running durability and proper head cleaning effect allowing the prevention of head stain be attained at the same time. JP-A-7-182646 proposes that a medium comprising Ba ferrite be reproduced as MR head. It is known that in the case where a medium is reproduced as MR head, when the product of the magnetization of magnetic tape and the thickness of the magnetic tape is not smaller than half the magnetization per unit volume of MR element, MR head undergoes saturation and hence deterioration of characteristics (see A. Okabe et al., IEEE Trans. MAG., 1996, MAG-32.3404), making it necessary that the magnetic tape be redesigned. It is thus necessary that the particle size of ferromagnetic powder to be incorporated in the magnetic layer, the abrasive, the filler for forming protrusions on the surface of the tape, the thickness of the magnetic layer, etc. be reviewed.
JP-A-3-286420 and IEEE Trans. MAG., November 1988, vol. 24 (6), p. 2850, etc. disclose that the anisotropic magnetic filed Hk of hexagonal ferrite has an effect on the electromagnetic characteristics of a magnetic recording medium. JP-A-3-286420 discloses a magnetic recording medium comprising two magnetic layers provided on a non-magnetic layer wherein the lower magnetic layer has a longitudinal axis of easy magnetization and the upper magnetic layer comprises a magnetic powder having an anisotropic magnetic field (Hk) of 239 kA/m or less incorporated therein, whereby a magnetic recording medium capable of producing a high output over a wide range of from short wavelength to long wavelength can be provided. JP-A-8-115518 proposes a high density recording medium having Hc of from 103.5 to 398 kA/m, Hc/Hk of from 0.30 to 1.0 and an in-plane squareness ratio SQ of from 0.65 to 1.00. The high density recording medium thus proposed is characterized in that the range of the various numeral values of Hc, Hc/Hk and in-plane squareness ratio SQ of the magnetic layer comprising hexagonal ferrite are predetermined, making it possible to drastically enhance the extremely short wavelength output required for high density recording. However, this high density recording medium is disadvantageous in that when used as MR head, the resulting noises are high.
In order to reduce noises from medium, there is a trend for size reduction of ferromagnetic particles. However, it is presumed that the size reduction of ferromagnetic particles will be accompanied by the effect of thermal fluctuation that affects the stability of the magnetization transition region. The stability of magnetization is evaluated by KuV/kT (in which Ku is a magnetic anisotropy constant, V is the volume of particle, k is Boltzmann's constant and T is absolute temperature). For the details of particle volume and thermal fluctuation of metallic tape, reference can be made to Toshiyuki Suzuki et al., “Shingaku Giho”, Nov. 21, 1997, MR97-55, P. 33–40.
Unlike ferromagnetic metal powder, hexagonal ferrite is not likely to undergo oxidation. However, hexagonal ferrite exhibits saturated magnetization of from about ⅓ to ½ of ferromagnetic metal powder. Accordingly, hexagonal ferrite can difficultly have a great Ku value and thus undergoes a great effect of thermal fluctuation.
It is said that a magnetic recording medium comprising hexagonal ferrite has a great mutual interaction between particles that affects the noise level of the medium. It is also said that when the mutual interaction between particles is great, the resulting stability of magnetization is excellent. However, it is likely that when inversion of magnetization occurs due to some reasons, the surrounding magnetic material, too, can undergo inversion of magnetization. Probably for this reason, it is disadvantageous in that desired C/N ratio can difficultly be sufficiently obtained when a high density recording medium formed by a hexagonal ferrite magnetic powder having a reduced particle size is reproduced as MR head. For the details of stability of magnetization of Ba ferrite medium, reference can be made to Tanaka and Suzuki, “Nihon Oyo Jiki Gakkaishi”, 2002, 26 (4), P. 258–262. Tanaka et al estimated the intensity of inversed magnetic field in a short period of time as used in head recording by the residual coercive force (Hr) using a vibrating sample magnetometer and a pulse magnetic field generator to estimate the effective KuV/kT. The estimation thus obtained indicates from 1.5 to 1.8 times Hr measured by the vibrating sample magnetometer, i.e., 422 to 478 kA/m. This means that magnetic recording can be difficultly made unless the head current is raised.